1. Field of the Invention
This invention relates generally to a method for planarizing wafer-based integrated circuits, and more particularly to a novel method for planarizing the surface of an integrated circuit having optical elements disposed thereon. Even more particularly, the invention relates to a novel method for planarizing the reflective surface of a wafer-based, reflective light-valve backplane.
2. Description of the Background Art
Wafer-based reflective light-valves have many advantages over their transmissive predecessors. For example, conventional transmissive displays are based on thin-film transistor (TFT) technology, whereby the displays are formed on a glass substrate, with the TFTs disposed in the spaces between the pixel apertures. Placing the driving circuitry between the pixel apertures limits the area of the display available for light transmission, and therefore limits the brightness of transmissive displays. In contrast, the driving circuitry of reflective displays is located under reflective pixel mirrors, and does not, therefore, consume valuable surface area of the display. As a result, reflective displays are more than twice as bright as their transmissive counterparts.
Another advantage of wafer-based reflective displays is that they can be manufactured with standard CMOS processes, and can therefore benefit from modem sub-micron CMOS technology. In particular, the reduced spacing between pixel mirrors increases the brightness of the display, and reduces the pixelated appearance of displayed images. Additionally, the CMOS circuitry switches at speeds one or more orders of magnitude faster than comparable TFT circuitry, making wafer-based reflective displays well suited for high speed video applications such as projectors and camcorder view finders.
FIG. 1 is a cross-sectional view of a prior art reflective display backplane 100, which is formed on a silicon substrate 102, and includes a layer 104 of integrated circuitry, an insulating support layer 106, a plurality of pixel mirrors 108, and a protective oxide layer 110. Each of pixel mirrors 108 is connected, through an associated via 112, to the circuitry of layer 104. Backplane 100 is typically incorporated into a reflective light valve (e.g., a liquid crystal display) by forming a layer 114 of an optically active medium (e.g., liquid crystal) over the pixel mirrors, and forming a transparent electrode (not shown) over the optically active medium. Light passing through the medium is modulated (e.g., polarization rotated), depending on the electrical signals applied to pixel mirrors 108.
One problem associated with prior reflective displays is that the generated images often appear mottled. One source of mottling in reflective displays is the non-uniform alignment of the liquid crystals in layer 114. The formation of liquid crystal layer 114 typically includes a wiping or rubbing step, wherein a roller or similar object is passed over the liquid crystal layer, resulting in alignment of the liquid crystals. However, pixel mirrors 108 project upward from the surface of backplane 100, defining gaps between adjacent pixel mirrors. Known wiping processes are ineffective to align the liquid crystals (represented by arrows in layer 114) in these gaps. Additionally, the misaligned crystals adversely affect the alignment of neighboring crystals in layer 114.
What is needed is a reflective backplane with a planar surface to facilitate the effective alignment of the entire liquid crystal layer.
FIG. 2 is a cross sectional view of a reflective backplane 200, illustrating the ineffectiveness of a method of planarizing the surface of reflective backplane 200 by depositing a thick protective oxide layer 202 and then etching layer 202 back to a desired thickness level 204. In particular, as oxide layer 202 is deposited, the opening 206 to the gap between pixel mirrors 108 closes before the gap is filled. This is known to those skilled in the art as the xe2x80x9ckeyhole effect.xe2x80x9d Then when oxide layer 202 is etched back to level 204, a nonplanar defect 208 remains over the partially filled gap, and will frustrate the uniform alignment of layer 114.
FIG. 3 is a cross sectional view of a reflective display backplane 300 illustrating an anticipated problem of using a prior art planarization process on a reflective backplane. Backplane 300 is planarized by forming a thick oxide layer 302 over pixel mirrors 108 and the portion of support layer 106 exposed by the gaps between pixel mirrors 108. Thick oxide layer 302 can only be formed without the keyhole effect shown in FIG. 2, if pixel mirrors 108 are sufficiently thin (e.g., less than 1000 angstroms). After its application, thick oxide layer 302 is ground down to a planar level 304, by a prior art process known as chemical-mechanical polishing (CMP). This process, also referred to as chemical-mechanical planarization, and is well known to those skilled in the art.
This method of planarizing the surface of reflective backplane 300 suffers from the disadvantage that it is limited to layers having a thickness of greater than or equal to 1,000 angstroms. That is, the CMP process is incapable of leaving an oxide layer of less than 1,000 angstroms over pixel mirrors 108. This planarization method is, therefore, not well suited for planarizing substrates having optical elements disposed on their surface, because the thickness of the film remaining on the optical element is often critical to its proper optical functionality.
What is needed is a method for planarizing the surface of substrates having optical elements disposed on their surface, that is capable of leaving layers over the optical elements, having a thickness of less than 1,000 angstroms.
The present invention overcomes the limitations of the prior art by providing a novel method for planarizing a substrate (e.g., a reflective display backplane) including a plurality of surface projections (e.g., pixel mirrors) and thin surface films (e.g., optical thin film coatings). Where the substrate is a reflective display backplane, the resulting planar surface reduces mottling in projected images.
A disclosed method includes the steps of forming an etchable layer on the substrate, performing a CMP process on the etchable layer to form a planar layer having a first thickness (e.g.,  greater than 1,000 Angstroms), and etching the planar layer to a second thickness (e.g.,  less than 1,000 Angstroms). In a particular method, the substrate is an integrated circuit and the projections are optical elements. In a more particular method, the substrate is a reflective display backplane, and the projections are pixel mirrors.
In a particular method suitable for planarizing substrates having surface projections in excess of 1,200 Angstroms, the step of forming the etchable layer includes the steps of forming an etch-resistant layer on the substrate, forming a fill layer on the etch-resistant layer, etching the fill layer to expose portions of the etch resistant layer overlying the projections and to leave a portion of the fill layer in the gaps, and forming the etchable layer on the exposed portions of the etch-resistant layer and the fill layer.
The etch resistant layer may include an optical thin film layer, and may be formed as a single layer. Optionally, the etch resistant layer includes a plurality of sublayers, for example an optical thin film layer and an etch resistant cap layer. In a particular method, the step of forming the etch resistant layer includes a step of forming an oxide layer and a second step of forming a nitride layer on the oxide layer.
The step of forming a fill layer on the etch resistant layer may include the step of applying a spin-on coating (e.g., spin-on glass) over the etch resistant layer. Optionally, the fill layer includes a suitable dopant to absorb light of a particular wave length.
One particular method of the present invention further includes an optional step of forming a protective layer over the planar layer. The protective layer may include a single layer or multiple layers. For example, in one more particular method, the step of forming the protective layer includes forming a nitride layer over the planar layer. Another more particular embodiment, further includes a step of forming an oxide layer over the nitride layer, and then forming a second nitride layer over the oxide layer.